1. Field of the Invention
This invention relates to a low-density polyethylene composition useful for coating polar substrates. This invention particularly relates to a low-density polyethylene composition useful for coating metal foils.
2. Description of the Related Art
Low density polyethylene (LDPE) can be used in packaging applications and/or other applications as a coating on polar substrates such as aluminum foil, paper, primed films, and metallized films, for example. LDPE can be useful as either a sealant, as an adhesive, or both, in these applications.
Other methods of coating LDPE on a polar surface are known, but typically LDPE can either be extrusion-coated or laminated onto a polar substrate. In such a process, the polymer is melted and coated as a curtainxe2x80x94that is as a thin, wide band of polymer melt carefully poured onto the substratexe2x80x94as the substrate is pulled across a cold chill roll at high speed. The LDPE freezes on the cold surface of the substrate. To promote adhesion in this high-speed process, the polymer curtain can be oxidized by application of high temperature with long exposure to air or ozone, flame, corona, or other treatments. The results are inconsistent and the strength of the adhesion is not reliable, and the resulting bond is typically weak. Other problems encountered while using this process are that the coated substrate can have poor organoleptic properties, there is off-gassing, smoke generation, polymer degradation, and other processing problems.
Acid copolymer resins (ACR""s), such as Nucrel(copyright), and ionomers such as Surlyn(copyright), available from E. I. DuPont de Nemours and Company can be used in place of LDPE to bond to polar substrates in extrusion coating or lamination processes. Resins such as these can bond well to foils in high line-speed processes having short exposure times and lower melt temperatures. These resins can include from 6 to 12 wt. % methacrylic acid or acrylic acid, and therefore can be more expensive than LDPE. Consequently, acid copolymers and ionomers are more typically used in the more demanding applications.
Some applications are moderately demanding, and can utilize acid copolymers having a lower acid content. For example, resin grades having only 4 wt % methacrylic acid, and only 3 wt % acrylic acid are available commercially for use in applications wherein only moderate performance is required of the adhesive. However, these resins can nevertheless be too expensive for most applications.
A resin with grafted anhydride functionality can be blended with LDPE and can improve adhesion to foil, but anhydride grafted resins do not bond to polar substrates well in high speed extrusion coating processes. Rubber and/or other elastomeric materials are typically added to resins having grafted anhydride functionality to improve adhesive properties. Polyolefins having acrylic acid grafted functionality of 6 wt % can be purchased commercially. These polymers cannot be processed at temperatures typical in an extrusion coating process, typically from 260xc2x0 C. to 320xc2x0 C. Blends of high acid copolymers or ionomers, having acid content of from 6% to 12%, with LDPE have been considered for improving adhesion of LDPE to foil because (1) high acid resin grades are normally used in extrusion coating, and (2) resins having high-acid content normally give better adhesion to foil than low acid resins. Surprisingly, however, in some cases blending high-acid acid copolymer resins with LDPE does not enhance the adhesion of LDPE to foil. More surprising in view of this is that blends of low acid copolymer resins with LDPE have significantly improved adhesion to polar substrates over LDPE.
In one aspect the present invention is a polymer blend, suitable for coating a polar substrate in a high-speed coating process, the blend comprising LDPE and a lowacid content acid copolymer resin (low-acid ACR), wherein the blend includes from about 5 to about 45% by weight low acid ACR and from about 95% to about 55% by weight LDPE, and wherein the low-acid ACR is an ethylene/(meth)acrylic copolymer comprising from about 0.1 to about 4 wt % (meth)acrylic acid and from about 0 to about 20 wt % other ethylenically unsaturated comonomer.
In another aspect the present invention is a high-speed extrusion process for coating polar substrates comprising the steps: melting a blend comprising a low-acid content acid copolymer resin (low acid ACR) and LDPE, wherein the polymer blend comprises from 5 to 45% by weight low acid ACR, wherein the low acid ACR is an ethylene/(meth)acrylic copolymer comprising from about 0.1 to about 4 wt % (meth)acrylic acid, and from about 0 to about 20 wt % other ethylenically unsaturated comonomer; extruding the polymer melt as a curtain through a polymer die onto a polar substrate, wherein the substrate is moving at a line speed in the range of from about 100 feet per minute (fpm) to about 2000 fpm; and pulling the polar substrate over a cold chill roll to solidify the polymer melt on the substrate.
In another aspect, the present invention is a high-speed extrusion lamination process for coating polar substrates comprising the steps: melting a blend comprising a low-acid content acid copolymer resin (low acid ACR) and LDPE, wherein the polymer blend comprises from 5 to 45% by weight low acid ACR, wherein the low acid ACR is an ethylene/(meth)acrylic copolymer comprising from about 0.1 to about 4 wt % (meth)acrylic acid, and from about 0 to about 20 wt % other ethylenically unsaturated comonomer; extruding the polymer melt as a curtain through a polymer die onto a polar substrate, wherein the substrate is moving at a line speed in the range of from about 100 feet per minute (fpm) to about 2000 fpm; sandwiching the polymer melt between the polar substrate and a second substrate to form a laminate, and pulling the laminate over a cold chill roll to solidify the polymer melt between the two substrates.
In still another aspect the present invention is a coated article wherein the article comprises a polar substrate and a polymer coat on at least a portion of one surface of the substrate, wherein the polymer coat is a blend comprising LDPE and a low acid ACR, wherein the blend includes from about 5 to about 45% by weight low acid ACR and from about 95 to about 55% by weight LDPE, and wherein the low acid ACR is a ethylene/(meth)acrylic copolymer comprising from about 0.1 to about 4 wt % (meth)acrylic acid, and from about 0 to about 20 wt % other ethylenically unsaturated comonomer.
In one embodiment, the present invention is a polymer blend useful for coating a polar substrate in a high-speed coating process. The blend comprises low density polyethylene (LDPE) and a low acid content acid copolymer resin (low acid ACR). Blends of the present invention include from about 5 wt % to about 45 wt % low acid ACR. Preferably blends of the present invention include from about 5 to about 40 wt % low acid ACR, more preferably from about 10 to about 35 wt % low acid ACR. Most preferably, blends of the present invention include form about 15 to about 30 wt % low acid ACR. Concomitantly, blends of the present invention include from about 95 wt % to about 55 wt % LDPE, preferably from about 95 to about 60 wt % LDPE. More preferably, blends of the present invention include from about 90 wt % to about 65 wt %, and most preferably from about 85 wt % to about 70 wt % LDPE.
LDPE can be manufactured by known processes, or can be obtained commercially. The LDPE is manufactured using a high pressure free radical polymerization process. Alternatively, LDPE suitable for use in the practice of the present invention can be obtained commercially from various manufacturers. For example, LDPE can be purchased from Equistar under the tradename of Petrothene. The LDPE sutiable for this invention is designed for the extrustion coating/lamination process, typically has a MI (melt index, ASTM D1238, Condition 190/2.16) from 4 to 15 g/10 min and density 0.915-0.925 g/cc. Other versions of polyethylene such as linear PE and linear PE with some levels of long chain branching produced in low pressure, catalysis processes, may also be suitable for this invention, provided they are suitable for processing in extrusion coating. Blends of linear and high pressure polyethylene may also be suitable for the invention. Typically high pressure LDPE, however, is preferred in the extrusion coating process because the presence of long chain branches in the polymer provide good processing at high line speeds.
Low acid ACRs of the present invention are copolymers of ethylene and at least one xcex1,xcex2-unsaturated carboxylic acid monomer, or a suitable derivative thereofxe2x80x94including ester derivatives. Low acid ACRs suitable for use herein comprise 4 wt % or less, based on the total weight of the polymer, of a carboxylic acid, preferrably either acrylic acid (AA), methacrylic acid (MAA), esters of maleic anhydride including the mono ethyl ester (MAME) of maleic anhydride, or a mixture of any of these. For the purposes of the present invention, the term (meth)acyrlic acid as used herein is shorthand notation to denote that any of these acids can be used either alone or in combination. Actual examples herein will refer to the specific acid, or the spcific combination of acids. Preferably, low acid ACRs of the present invention include from about 0.1 to about 4 wt % (meth)acrylic acid, more preferably from about 0.5 to 3.5 wt % (meth)acrylic acid, and most preferably from about 1.0 to about 3.0 wt % (meth)acrylic acid.
Optionally, a low acid ACR of the present invention can include other ethylenically unsaturated comonomers in addition to (meth)acrylic acid. Other comonomers suitable for the practice of the present invention include: isobutylacrylate (iBA); n-butylacrylate (nBA); methyl acrylate; ethyl acrylate. Also suitable for use herein are xcex1,xcex2-unsaturated diacids and their derivatives, such as for example: itaconic acid; maleic acid; fumaric acid; monoalkyl esters of any of these, such as methyl hydrogen maleate; anhydrides such as maleic anhydride; like compounds, or mixtures of any of these. A low acid ACR of the present invention can include from 0 to 20 wt % of other comonomers. Preferably, a low acid ACR of the present invention can include from about 1 to about 10 wt % of other comonomers, more preferably from about 1 to about 8 wt %, and most preferably from about 3 to about 7 wt % of other comonomers.
In a preferred embodiment, the present invention is a polymer blend as described hereinabove, wherein the low acid ACR is an ethylene/(meth)acrylic acid/iBA terpolymer that includes from about 1 to about 8 wt % of iBA. More preferably, the terpolymer includes from about 3 to about 7 wt % iBA, most preferably about 6 wt % iBA.
Acid copolymer resins of the present invention can be obtained by directly copolymerizing ethylene and (meth)acrylic acid and, optionally, other comonomers according to methods known in the art of manufacturing acid copolymer resins. Copolymers having grafted acid functionality are not suitable for the practice of the present invention. For example, an acid copolymer resin suitable for use in the practice of the present invention can be obtained by a high pressure free radical polymerization process. Low acid ACR of the present invention have melt index (MI), as determined according to ASTM D1238 (Condition 190xc2x0 C./2.16 kg) of from about 2 to about 50 g/10 min. Preferably the MI is from about 6 to about 15 g/10 min, more preferably from about 6 to about 13 g/10 min, and most preferably from about 7 to about 12 g/10 min.
A low acid ACR suitable for use in the practice of the present invention and LDPE can be blended in the melt prior to the extrusion coating/lamination process using a single screw extruder, twin screw extruder, or other compounding device known in the art. Alternatively, the low acid ACR and LDPE can be blended together as pellets and introduced into the hopper of the extrusion coating/lamination extruder. Standard screws for extrusion coating can be sufficient to achieve mixing of the two resins.
Processing aides known in the art of polymer extrusion/fabricationxe2x80x94such as chill roll release agents, slip agents, antiblock agents, and the likexe2x80x94may be added to the blend in small amounts, as practiced in the art. Additives known to negatively affect the adhesion of LDPE, such as antioxidants, should be kept to a minimum.
In another embodiment, the present invention is a process for coating a polar substrate with a polymer blend, wherein the blend comprises LDPE and a low acid ACR, as described hereinabove. A process of the present invention can be either an extrusion (or co-extrusion) process or an extrusion (or co-extrusion) lamination process. In either case, the process of the present invention is a high line speed (high-speed) process, wherein the subtrate is passed through an extrusion coater at a high velocity. A high speed extrusion process of the present invention comprises the steps: melting the polymer blend; extruding the polymer through an extrusion die; applying the polymer melt onto a polar substrate; and pulling the polymer melt-coated substrate over a chill roll at a rapid speed, such that the polymer melt is in direct contact with the chill roll, to obtain the coated polar substrate. In a high-speed extrusion lamination process, the polymer melt is sandwiched between two substrates, the polar substrate and a second substrate, such that the second substrate is in direct contact with the chill roll. The second substrate can be either polar or non-polar.
A suitable polymer melt can be obtained from a blend of LDPE and low acid ACR as described hereinabove, at a temperature above about 250xc2x0 C. Preferably, the polymer melt is obtained at a temperature of from about 250xc2x0 C. to about 335xc2x0 C. More preferably, the polymer melt is obtained at a temperature of from about 260xc2x0 C. to about 330xc2x0 C., and most preferably from about 270xc2x0 C. to about 320xc2x0 C.
In the high-speed extrusion coating process of the present invention, the substrate is coated at a line speed of from about 100 fpm (feet per minute) to about 2000 fpm in. Preferably, the substrate is coated at a line speed of from about 100 fpm to about 1500 fpm, more preferably from about 150 to about 1400, and most preferably from about 200 to about 1300 fpm.
Maintaining a proper air gap can be critical to the process of the present invention. In the process described herein, a suitable air gap can be from about 3 to about 15 inches. Preferably the air gap is from about 3 to about 15 inches, more preferably from about 4 to about 10 inches, and most preferably from about 4.5 to about 9.5 inches.
The line speed and air gap can be combined to give a parameter known as xe2x80x9ctime in the air gapxe2x80x9d (TIAG). TIAG=L/v, wherein L is the air gap and v is the line speed. In the practice of the present invention, TIAG is from about 7.5 to about 300 miliseconds (ms), preferably from about 30 ms to about 200 ms, more preferably from about 50 ms to about 150 ms, and most preferably from about 60 ms to about 120 ms. The preferred thickness of a coating of the present invention can depend on the application for which the coated substrate is being prepared. However, the blended polymer melt can be extruded through an extrusion die at a suitable temperature as described hereinabove, in any case a temperature above the melt temperature of the polymer blend, such that a coating of thickness less than about 5 mil is applied to the surface of the substrate. Preferably coating thickness is less than about 2, more preferably the coating thickness is less than about 1.5. The polymer melt can be combined with another polymer in a process called coextrusion coating/lamination whereby the thickness of the blended layer can be considerably less than in an extrusion coating process. Alternatively, multiple layers can be laid down by consecutive overcoatings in a process known in the art of extrusion coating as tandem coating.